1. Field of the Invention
This invention relates to the field of tunable inductors, particularly those which can be integrated.
2. Description of the Related Art
Inductors are found in innumerable electronic circuits, and are particularly prevalent in RF circuitry. The performance of many RF systems depends critically on how precisely specific inductance values can be provided in a circuit, and on the quality factor or "Q" of the circuit's reactive components. Q for an inductor is defined as the maximum amount of energy stored in the magnetic field of the inductor's coils divided by the amount of energy lost by the inductor during one complete cycle of a signal applied to the inductor; for an inductor, Q is given by: ##EQU1## where .omega.=2.pi.f and f is the frequency of the signal applied to the inductor, L is the inductor's inductance value and R is the inductor's resistance.
As is evident by Equation 1, a high resistance value lowers an inductor's Q. This is particularly important when the inductor is employed in a resonant circuit, where the value of Q is directly related to the sharpness of the circuit's frequency response. Many resonant circuits are deliberately designed with high-Q inductors to take advantage of the narrow bandwidth and high frequency selectivity associated with their use.
The inductance value L of an inductor is also important in determining the frequency .omega..sub.0 at which a resonant circuit resonates. The resonant frequency .omega..sub.0 of a parallel LC circuit is given by: ##EQU2## Thus, the frequency response of a resonant circuit peaks at a frequency determined by the circuit's inductance and capacitance values, and the width of the peak depends on the Q value of the circuit's components. Resonant circuits are discussed, for example, in Hayt and Kemmerly, Engineering Circuit Analysis, McGraw-Hill, Inc. (1971), pp. 396-408.
Obtaining high-Q inductors with precise inductance values has traditionally been accomplished by either hand-selecting an inductor having desired characteristics from a batch, or by trimming the inductor as needed after manufacture. However, even state-of-the-art laser trimming methods impose limits on how closely one can get to a desired inductance value, and both hand-selecting and trimming are expensive and labor-intensive.
It is often desirable to fabricate inductors with integrated circuit techniques. Integration enables a circuit's inductors to be made simultaneously with other circuit components, reduces the distance between a circuit's inductors and its other components, eliminates the need for parasitic capacitance-causing wire bonds, and reduces the circuit's space and weight requirements, which are typically at a premium in wireless communications devices. However, integrated inductors are difficult to trim to specific inductance values and require a considerable amount of die area to produce a significant amount of inductance.
One method of providing a precise inductance value in an I.C. requires a number of fixed inductors to be fabricated on a substrate, which are then selectably interconnected with solid-state or off-chip switches to produce a desired value of inductance. However, there are several problems related to the use of solid-state switches, particularly at microwave frequencies and above. Integrated switches capable of handling microwave frequencies are typically implemented with gallium arsenide (GaAs) MESFETs or PIN diode circuits. At signal frequencies above about 900 MHz, such as those used by a cellular phone, these switching devices or circuits typically exhibit an insertion loss in the `ON` (closed) state of about 0.5 db. These shortcomings practically limit the number of inductors that can be interconnected when the losses incurred by the switches becomes unacceptably high. Additional gain must often be built into a system to compensate for the poor performance of the devices, increasing power dissipation, cost and increasing unit size and weight. The characteristics of GaAs MESFETs and PIN diode switches are discussed, for example, in R. Dorf, The Electrical Engineering Handbook, CRC Press (1993), pp. 1011-1013.
Providing switching with PIN diode circuits presents additional problems due to the parasitic capacitances inherently created by their use, which serve to limit the frequency range over which the circuit can operate. Similar problems arise when the necessary switching is provided by off-chip switches, due to the parasitic capacitances that result from the presence of wire bonds.
Another major drawback of using state-of-the-art solid state switches is that they place a considerable amount of resistance in series with the inductor, often severely lowering its Q. A low-Q inductor causes the frequency response of the circuit in which it is used to flatten out, lowering its selectivity and widening its bandwidth, often rendering the circuit impractical for use in wireless communications devices.